In recent years, switching power supply apparatuses have been required to be more compact, efficient and reliable as electronic apparatuses in which switching power supply apparatuses become less expensive, more compact, higher in performance and more energy-efficient. A switching power supply apparatus basically comprises an inverter that turns on and off an input DC voltage at a high frequency to covert it into a high-frequency AC voltage, a transformer for converting AC currents and voltages and for keeping insulation between input and output, and a rectifying and smoothing circuit for converting an alternating current into a direct current. The output voltage is regulated by changing the on/off ratios of the switching devices in the switching sections of the inverter section. Since the transformer is driven at a high frequency, it can be made compact. In addition, the transformer has an extremely low loss because of on/off operation. For these reasons, the switching power supply apparatus is characterized to be made compact and highly efficient.
However, in reality, there is no ideal switching device. In a semiconductor switch used in a switching section or a rectifying circuit, an on-voltage occurs due to the resistance of the semiconductor switch at its on-period, thereby causing a loss. Since a high-speed on/off characteristic is usually required for a rectifying and smoothing circuit in particular, high-speed diodes are used for the circuit. However, the high-speed diode has the disadvantage of requiring a particularly high on-voltage, thereby reducing efficiency. In recent years, MOS FETs have been improved greatly in performance, and attempts have been made to improve their efficiency by using such MOS FETs as rectifying devices and by carrying out synchronous rectification described below.
Half-bridge-type and full-bridge-type circuits have been used for high-power switching power supply apparatuses. FIG. 6 shows a circuit diagram of a conventional example of a switching power supply apparatus comprising a combination of a full-bridge converter and synchronous rectifying devices. Referring to FIG. 6, the voltage value of an input DC power supply 1 is assumed to be "Vin". A series circuit comprising a first switching section 69 and a second switching section 70 is connected across input terminals 2a and 2b. A series circuit comprising a third switching section 71 and a fourth switching section 72 is also connected across the input terminals 2a and 2b. A transformer 73 has a primary winding 73a, a first secondary winding 73b, a second secondary winding 73c, a first driving winding 73d and a second driving winding 73e. The ratio of the number of turns of the above-mentioned five windings is represented by N:1:1:N':N' (N is a positive number). One terminal of the primary winding 73a is connected to the connection point of the first switching section 69 and the second switching section 70, and the other terminal is connected to the connection point of the third switching section 71 and the fourth switching section 72.
The cathode of a first rectifying diode 74 and the cathode of a second rectifying diode 75 are connected to each other, and their anodes are connected to the first secondary winding 73b and the second secondary winding 73c of the transformer 73, respectively. A fifth switching section 76 is connected in parallel with the first rectifying diode 74, thereby forming a synchronous rectifying circuit. By connecting the fifth switching section 76, i.e., a switching device, the on-voltage of which is lower than that of the first rectifying diode 74, in parallel with the first rectifying diode 74, the loss in this configuration can be made lower than that in the case when only the rectifying diode 74 is used. The fifth switching section 76 operates so as to turn on only when a positive voltage generates at the first driving winding 73d of the transformer 73. A sixth switching section 77 is connected in parallel with the second rectifying diode 75. The sixth switching section 77 and the second rectifying diode 75 operate so that the sixth switching section 77 turns on only when a positive voltage generates at the second driving winding 73e of the transformer 73. This forms a synchronous rectifying circuit.
An inductance device 16 is connected in series with a smoothing capacitor 17. One terminal of this series circuit is connected to the connection point of the first secondary winding 73b and the second secondary winding 73c, and the other terminal is connected to the connection point of the first rectifying diode 74 and the second rectifying diode 75. Output terminals 18a and 18b are connected across both terminals of the smoothing capacitor 17. A load 19 is connected across the output terminals 18a and 18b, and consumes electric power. A PWM circuit 20 detects a voltage "Vout" across the output terminals 18a and 18b, and generates a PWM signal that controls the voltage constant. A distribution circuit 21 distributes the signal supplied from the PWM circuit 20 to two channels. A first high side driving circuit 78 and a second high side driving circuit 81 are each formed of a semiconductor device or a driving transformer, and generate on/off signals for turning on/off the first switching section 69 and the third switching section 71, respectively depending on the output of the distribution circuit 21. "High side" refers to the positive side of the input DC power supply 1.
A first driving circuit 80 and a second driving circuit 79 generate on/off signals for turning on/off the second switching section 70 and the fourth switching section 72, respectively depending on the output of the distribution circuit 21. Since the first high side driving circuit 78 and the second driving circuit 79 have a common input, the first switching section 69 and the fourth switching section 72 turn on/off simultaneously.
In the same way, since the second high side driving circuit 81 and the first driving circuit 80 have a common input, the second switching section 70 and the third switching section 71 turn on/off simultaneously. A third driving circuit 82 generates an on/off signal so that the fifth switching section 76 turns on when a positive voltage generates at the first driving winding 73d of the transformer 73. In the same way, a fourth driving circuit 83 generates an on/off signal so that the sixth switching section 77 turns on when a positive voltage generates at the second driving winding 73e of the transformer 73.
The operation of the switching power supply apparatus configured as described above will be described below referring to FIG. 7a to FIG. 7j. FIG. 7a to FIG. 7j show the waveforms of the signals, voltages and currents at relevant parts. FIG. 7a shows the on/off signals G1 and G4 of the first and fourth switching sections 69 and 72, respectively. FIG. 7b shows the on/off signals G2 and G3 of the second and third switching sections 70 and 71, respectively. FIG. 7C shows the current Ip flowing through the primary winding 73a of the transformer 73. FIG. 7d shows the voltage Vp applied to the primary winding 73a of the transformer 73. FIG. 7e shows the voltage Vsr1 generated at the first driving winding 73d of the transformer 73. FIG. 7f shows the voltage Vsr2 generated at the second driving winding 73e of the transformer 73. FIG. 7g shows the current Isr1 flowing through the fifth switching section 76. FIG. 7h shows the current Isr2 flowing through the sixth switching section 77. FIG. 7i shows the current Ir1 flowing through the first rectifying diode 74. FIG. 7j shows the current Isr2 flowing through the second rectifying diode 75.
When the first switching section 69 and the fourth switching section 72 are turned on by the on/off signals of the first high side driving circuit 78 and the second driving circuit 79, respectively at time t0, the input voltage Vin is applied to the primary winding 73a of the transformer 73. When the voltage Vin is applied to the primary winding 73a of the transformer 73, voltages are induced at the windings of the transformer 73. A positive voltage Vin/N*N' is generated at the first driving winding 73d. By properly selecting a value N', the fifth switching section 76 can be turned on. The voltage Vin/N induced at the first secondary winding 73b is applied to the series circuit of the inductance device 16 and the smoothing capacitor 17 via the fifth switching section 76 having been turned on. Since the current flowing through the inductance device 16 flows through the secondary winding 73b, a current flows through the primary winding 73a. Since the fifth switching section 76 has been selected to turn on at a sufficiently low voltage, no current flows thorough the first rectifying diode 74 connected in parallel therewith. The current flows through the fifth switching section 76 used as a synchronous rectifying device, whereby the rectifying diode 74 can have a lower rectifying loss.
When the first switching section 69 and the fourth switching section 72 are turned off by the off-signals of the first high side driving circuit 78 and the second driving circuit 79, respectively at time t1, the current Ip at the primary winding of the transformer 73 becomes zero. At this time, a turn-off surge voltage is generated by the energy stored in the leak inductance parasitically existing in the transformer 73, and this causes a loss. In addition, when the second rectifying diode 75 turns on, the current flowing through the inductance device 16 is divided and flows through the first secondary winding 73b and the second secondary winding 73c of the transformer 73 because of the continuity of the magnetic flux at the transformer 73. At this time, the secondary windings 73b and 73c of the transformer 73 are short-circuited by the first rectifying diode 74 and the second rectifying diode 75 having been turned on, whereby the voltages induced at the windings of the transformer 73 become zero. The voltages at the first driving winding 73d and the second driving winding 73e also become zero, whereby the fifth switching section 76 and the sixth switching section 77 turn off. At this time, the current flowing through the inductance device 16 flows through the first rectifying diode 74 and the second rectifying diode 75.
When the second switching section 70 and the third switching section 71 are turned on simultaneously by the on-signals of the first driving circuit 80 and the second high side driving circuit 81, respectively at time t2, the input voltage Vin is applied to the primary winding 73a of the transformer 73 in the opposite direction. Voltages depending on the turn ratio are induced at the windings of the transformer 73. A positive voltage generates at the second driving winding 73e of the transformer 73 and turns on the sixth switching section 77. At the first secondary winding 73b of the transformer 73, a voltage -Vin/N generates and turns on the first rectifying diode 74. The voltage generated at the second secondary winding 73c of the transformer 73 is applied to the series circuit of the inductance device 16 and the smoothing capacitor 17 via the sixth switching section 77 having been turned on. Hereafter, the same operation as that described above is repeated in the next half period.
The output voltage is controlled as described below. The on-periods of the first switching section 69 and the fourth switching section 72 are made equal to the on-period of the second switching section 70 and the third switching section 71, and is set at Ton (=t1-t0=t3-t2). The off-period thereof is set at Toff (=t2-t1=t4-t3). When a period Ts is assumed to be Ts=Ton+Toff, since the average of the voltage applied to the inductance device 16 becomes zero in the steady state, the output voltage Vout is represented by Vout (=1/N (Ton/Ts)). Therefore, the output voltage can be controlled by changing the on/off ratios of the switching sections.
This control can turn on the fifth switching section 76 and the sixth switching section 77, which carry out synchronous rectification only in the period when a voltage is applied to the transformer 73, whereby voltage drop can be reduced and efficiency can be improved. In addition, when no voltage is generated at the first secondary winding 73b and the second secondary winding 73c of the transformer 73, the fifth switching section 76 and the sixth switching section 77 remain off. Therefore, no short-circuit current flows at the moment when a voltage is applied to the primary winding 73a of the transformer 73.
In the above-mentioned descriptions, a full-bridge converter has been taken as an example to explain. However, the above-mentioned operation can also be applied similarly to synchronous rectifying switching power supply apparatuses using half-bridge converters and push-pull converters.
In the above-mentioned conventional configuration, however, the fifth switching section 76 and the sixth switching section 77, synchronous rectifying devices, cannot be driven in the period when no voltage is generated at the transformer 73. Furthermore, currents flow through the first rectifying diode 74 and the second rectifying diode 75 in this period, thereby causing a problem of increasing a loss. Therefore, if a control is carried out so that the input voltage rises and the on-periods of the first to fourth switching sections 69, 70, 71 and 72 are shortened, the periods during which currents flow through the fifth switching section 76 and the sixth switching section 77 shorten for carrying out synchronous rectification. As a result, this causes a problem of reducing the effect of synchronous rectification. In the same way, even if the on/off signals to be applied to the first to fourth switching sections 69, 70, 71 and 72 are used, the fifth switching section 76 and the sixth switching section 77 cannot be turned on when the first to fourth switching section 69 to 72 are off.
Generally speaking, when a MOS FET is used as a synchronous rectifying device, the body diode built in the MOS FET is used frequently as a rectifying diode. However, since the body diode turns on at a higher voltage, the effect of the synchronous rectification is reduced further. In addition, if it is turned on forcibly when no voltage is generated at the transformer 73, even if the switching devices on the primary side turn on and a voltage is applied to the transformer 73, a short-circuit condition is maintained until voltages are generated at the secondary windings of the transformer 73. This causes a problem of flowing an excessively large current. Furthermore, at the turn-on time of each switching section, a capacitor parasitically existing across the switching section is short-circuited. As a result, the energy stored in this capacitor causes a loss, and a surge-like short-circuit current causes noise and reduces reliability. Moreover, at the turn-off time of the switching section, the energy stored in the leak inductance of the transformer 73 causes resonance at the capacitor parasitically existing at the switching section, thereby also causing problems of occurrence of a loss and low reliability.
Accordingly, an object of the present invention is to provide a highly efficient, reliable switching power supply apparatus.